Cavity wall insulation - What for and how much?

Two leaves or skins made of bricks with insulation between them

Why is cavity insulation one of the most popular ways to insulate your home? Are there different types? How does it work exactly? We’ve investigated to bring you all the nitty-gritty details on cavity insulation in Ireland 2019.

A well-insulated home is crucial to reducing your household’s carbon footprint, not to mention reducing your heating bills, so find out whether cavity wall insulation is a suitable addition to your home. For information on all the different ways you can insulate your home, check out our guide on insulation.


What is a cavity wall?

A cavity wall is a type of wall that is hollow in the centre. The wall material on either side of the cavity is often referred to as  “skins”, or “leaves” and is usually made of bricks or blocks. You can normally tell if your home has cavity walls or not by looking at any section of exposed brickwork on the outside walls.

If all the bricks are laid evenly lengthways in a simple alternating pattern, you have a cavity wall. If, however, you can see the ends of some bricks as they have been layed widthways, then you have a solid wall. Solid walls are rare nowadays and are usually only found with pre-1920’s properties.

If you cannot see any exposed brickwork then measure how wide your external walls are. Walls more than 26cm thick are most likely to be cavity walls. 

If you’re still unsure as to whether your walls are cavity walls or not, or whether the cavity gas been filled, you can arrange an appointment with a cavity wall insulation installer. The installer will then drill a small hole in your wall to check whether there is a cavity, and if it is empty or not.


What is cavity wall insulation?

Cavity wall insulation is when a material is used to fill the space in between the two “skins”. The material inserted can be, but is not limited to:

  • Glass wool
  • Rock wool
  • Cellulose
  • Foam
  • Polystyrene beads

Most modern buildings are already built with insulated cavity walls, but older buildings do not. Almost a third of heat lost in uninsulated homes escapes through the walls. So while you should definitely look at installing nsulation everywhere in your home if it's an older build, cavity wall insulation is still pretty much a must.


How does cavity wall insulation work?

The main aim of cavity wall insulation is to reduce heat loss that may occur when the cavity between the skins is empty. Filling the space in a cavity is done to reduce the possibility of convection

What is convection?Convection is heat transfer which occurs when warmth and cold meet. Cold air tends to drop down while warmer air rises, creating a circular motion and heat loss.

By removing the possibility of air moving around between the two leaves of a wall, convection can be greatly reduced or even eliminated. To what degree depends on the quality of material used. The less air that is able to circulate, the less heat that can be lost.

To get the material into the cavity, it is normally injected into the wall. Small holes are drilled at gaps of about 1 metre in the wall and the insulation is injected through the holes. The holes are then sealed with cement or white putty. Installers are normally careful in their placement so as not to destroy the aesthetic of your house, but you can always touch up the filled-in holes with paint to match the surrounding walls.

It is a simple process which will only take around two hours . We do not recommend doing it yourself, due to the potential to cause structural or cosmetic damage to your outer walls. Note that if your house has narrow cavities or is in an area with a risk of flooding, you may not be able to have mineral wool or beads installed and may have to opt for the more expensive polystyrene foam insulation.


How much does cavity wall insulation cost?

bills and coins

The cheapest houses to install cavity insulation in, tend to be terraced houses, followed by semi-detached dwellings, with the final category of detached houses being the most expensive. The reason being that terraced houses only have two exposed walls to be filled, semi-detached have three, and a detached home has four.

Depending on which company you choose, cavity wall insulation can normally cost from €7.50 - €10.50 per m2. For a three-bedroom semi-detached house, this could mean anything from €600-€1000. 

If you can’t afford to have all of your outer cavity walls filled in one go, you can still do it on a wall-by-wall basis. Heat loss will not be entirely curbed by installing insulation piecemeal, but it can still be lessened.

Note that cavity wall insulation qualifys for an SEAI grant of up to €400.


How much money can cavity wall insulation save you?

Keep in mind that by installing insulation you should also be able to save up to €300 a year off your heating bills, so view cavity insulation as an investment that will pay off after just a couple of years.

Of course, there is no need to stop at just insulating your walls - your roof is also easy and cheap to insulate and will save you even more money. To save even more money, try some of our 100 tips to save money on your energy bills.


Cavity insulation is one of the most common upgrades for older houses in need of an energy makeover. The reasons for this are quite simple: cavity insulation is one of the cheapest and fastest ways to improve the BER (Building Energy Rating) of a house. 

For just two hours of work and a couple of hundred euro, you can instantly have a warmer house with less heat loss, and save enough money on heating to recover the cost within three years.


Are there any drawbacks to getting cavity wall insulation?

Aside from paying to get it done, there are no real drawbacks to cavity wall insulation. If there are mistakes made or problems encountered during the installation, then you may have some issues afterwards such as:

  • Damp
  • Mould

If you think these issues have been caused by the cavity wall insulation, then it’s best to contact the installer as soon as possible so they can arrange to fix the problem. If the issue has arisen because of the installation, then they should offer to fix it free of charge.

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